A molded case circuit breaker (MCCB) can incorporate a pressure sensitive trip mechanism, sometimes called a piston trip, to detect over current events and trip the breaker. Internal to the MCCB, a chamber houses two electrical contacts that are configured to separate due to electrodynamic forces generated when the current flowing through the contacts is excessively high. When the contacts separate, an arc occurs as the air between the contacts ionizes and electrical energy arcs between the contacts. The energy released during the arc heats the gas in the chamber and increases the pressure within the chamber. The chamber housing the contacts is sometimes referred to as a breaking unit. The breaking unit is in fluid communication with a piston trip pressure sensitive unit, which is another chamber that includes a movable surface that moves in response to the pressure increase communicated from the breaking unit. In some breakers, the movable surface is a piston moving within a cylinder. In others, the movable surface is one side of a lever that pivots when the pressure increases. The movement of the movable surface then activates a trip mechanism through a mechanical linkage. The trip mechanism can be configured to break multiple poles of an electrical circuit simultaneously. Such an MCCB generally incorporates exhaust vents for venting the high pressure gas following the activation of the trip mechanism.
An MCCB incorporating a pressure sensitive trip mechanism (also referred to as a piston trip module) generally incorporates a bias for biasing the movable surface in a normal operating position. A piston trip module incorporating a bias is disclosed in U.S. Pat. No. 5,298,874 to Morel et al. A spring can be used to bias the movable surface. During the arc, the movable surface moves against the force of the bias to activate the trip mechanism due to the high pressure created by the heated gas. Once the trip mechanism is activated, the arc halts. With the gasses no longer heated, the pressure in the breaking unit returns to normal. The return of normal pressure may be assisted by venting the heated gas into exhaust vents. After the pressure has stabilized, the bias causes the movable surface to return to the normal operating position.
Occasionally, however, the interior surface that the movable surface moves along is damaged during the arc fault event by hot gasses and molten metallic debris generated during the arc. Hot gasses and debris can become imbedded in the interior surface or otherwise foul the interior surface. The damage to the interior surface can impede the movement of the movable surface as it is returned to its normal operating position under the force of the bias. When the force of the bias is unable to return the movable surface to its normal operating position due to the fouled interior surface, the MCCB may trip while operating.